Recent developments of the 3rd Generation Partnership Project, 3GPP, Long Term Evolution, LTE, facilitate accessing local IP-based services in various places, such as at home, office, or public hot spots, or even in outdoor environments. One of the use cases for the local IP access and local connectivity involves a so-called D2D communication mode, wherein wireless devices, such as for example user equipments, UEs, in close proximity (typically less than a few tens of meters, but sometimes up to a few hundred meters) of each other communicate with each other directly.
Because D2D wireless devices may be closer to each other than cellular UEs that have to communicate via at least one cellular access point (e.g. a Radio Base Station, RBS, such as an evolved Node B, eNB), the D2D communication enables a number of potential gains over the traditional cellular technique, including capacity gain, peak rate gain, and latency gain.
The capacity gain may be achieved, for example, by reusing radio resources (e.g. Orthogonal Frequency Division Multiplexing, OFDM, resource blocks) between D2D and cellular communications and by reducing the number of links between wireless devices such as UEs from two to one and accordingly reducing the radio resources required for one link. The peak rate gain directly results from the relatively short distance between D2D UEs and the potentially favourable propagation condition there between. The latency gain is also a direct result of the single relatively short link between D2D UEs.
FIG. 1a illustrates an example of a mixed cellular and D2D network, wherein wireless device 101 is a cellular UE which communicates via an eNB 110, whereas wireless devices 102 and 103 are D2D wireless devices which communicate with each other directly. In such a mixed cellular and D2D network, D2D communications share radio resources with UL cellular communications, and a Time Division Duplex (TDD) is used as the duplex scheme for the bi-directional D2D communications.
For a pure cellular system using a Frequency Division Duplex, FDD, scheme, UL reception timings at an eNB are aligned for cellular subframes transmitted from all cellular wireless devices served by the eNB, while DL transmission timings are aligned with the UL reception timings, as illustrated at the top of FIG. 1b. In FIG. 1b, the wireless devices are denoted terminal 1 and terminal 2. Examples of a wireless device are a UE, a mobile telephone, a mobile station, a laptop, a personal digital assistant, PDA, and any other portable device or terminal having communication means enabling the device or terminal to communicate wirelessly with any other device, terminal or communication node.
In order to achieve the alignment of DL transmission and UL reception timings at the eNB side, in DL, each cellular wireless device receives a synchronisation signal from the eNB, and adjusts its reception timing according to the received synchronisation signal, so that the reception timing for a subframe at the wireless device coincides with the transmission timing for the subframe at the eNB plus a propagation delay, TP, from the eNB to the UE. In the middle and at the bottom of FIG. 1b, the TPs for a wireless device (denoted terminal 1 in FIG. 1b) close to the eNB and a wireless device (denoted terminal 2 in FIG. 1b) far from the eNB are respectively denoted as TP,1 and TP,2.
In UL, each wireless device receives from the eNB a timing advance, TA, calculated by means of random access channel, RACH, procedure and/or based on UL demodulation reference signal, DMRS, estimation, and adjusts its UL transmission timing in advance of its DL reception timing according to the TA. In the middle and at the bottom of FIG. 1b, the TAs for the terminal 1 close to the eNB and the terminal 2 far from the eNB are respectively denoted as TA,1 and TA,2.
In the mixed cellular and D2D network, a wireless device or UE may operate as a D2D receiving, RX, UE to receive data from its corresponding D2D transmitting, TX, UE and/or operate as a D2D TX UE to transmit data to its corresponding D2D RX UE, in addition to receiving and transmitting data from and to an eNB.
Cellular systems often define multiple states for the terminal matching different transmission activities. In LTE, two states are defined:                RRC_IDLE, where the wireless device is not connected to a particular cell and no data transfer in either uplink or downlink may occur. The wireless device is in DRX most of the time except for occasionally monitoring the paging channel.        RRC_CONNECTED, where the wireless device is connected to a known cell and can receive downlink transmissions. Although expressed differently in the specifications, it can be thought to have two “sub-states”:                    UL_IN_SYNC, where the wireless device has a valid timing advance value such that uplink transmissions can be received without collisions between different wireless devices.            UL_OUT_OF_SYNC, where the wireless device does not have a valid timing advance value and hence cannot transmit data in the uplink. Prior to any transmission, a random access must be performed to synchronise the uplink.                        
In LTE, random access is used to achieve uplink time synchronisation for a wireless device which either has not yet acquired, or has lost, its uplink synchronisation. Once uplink synchronisation is achieved for a wireless device, the eNB can schedule orthogonal uplink transmission resources for it. Relevant scenarios in which the RACH is used are therefore:                1) A wireless device in RRC_CONNECTED state, but not uplink-synchronised, needing to send new uplink data or control information (e.g. an event-triggered measurement report or a hybrid ARQ acknowledgement in response to downlink data transmission);        2) A wireless device in RRC_CONNECTED state, handing over from its current serving cell to a target cell;        3) For positioning purposes in RRC_CONNECTED state, when timing advance is needed for wireless device positioning;        4) A transition from RRC_IDLE state to RRC_CONNECTED, for example for initial access or tracking area updates;        5) Recovering from radio link failure.        
For D2D communication, it is necessary to define the transmission and reception timing. In principle, any transmission timing could be used as long as transmissions do not interfere with cellular communication. However, an attractive approach for D2D communication is (especially for broadcast type communication) to require the D2D TX to be RRC connected, but allow RRC idle UEs to receive.
In other words, to use the same transmission timing at the D2D TX for D2D transmissions as for cellular uplink transmissions. This ensures that D2D transmissions do no collide with uplink transmissions from the same device and avoids a (potentially complicated) additional timing advance mechanism for direct D2D communication.
So a problem is how to enable the RRC_IDLE UEs to receive data. As an RRC_IDLE UE, as stated above, only has DL timing, but no information of UL timing, i.e. TA value. In this case, it cannot receive the D2D TX signal from another device since:                Considering the propagation delay within each D2D link, the timing difference is 2*link length, e.g., we need to handle 2*500 m/c=3.3 μs (the D2D link length maybe larger than 500 m in an extreme case); c is the speed of light equal to 3*108 m/s;        Considering the propagation delay between D2D TX and eNB, the timing difference between UL timing (at TX side) and DL timing (at Rx side) is 2*UE-eNB link length, e.g., we need to handle 2*1 km/c=6 μs.        
So to handle this (6+3.3) μs difference (plus the channel delay spread), it is not enough to use the traditional normal (Cycic Prefix) CP-5 μs length, since this would cause reception failure at the D2D Rx.
Even though one may argue that we can use extended CP (16 μs), it means:                Less legacy support on eNBs: normal CP is a widely used case, while extended CP is mostly limited to evolved Multimedia Broadcast Multicast Service (eMBMS) scenario;        Limited to intra-cell scenario: For un-sync eNBs scenario, the timing difference between neighbouring cells is not predicted, so even 16 μs will not guarantee inter-cell D2D communication;        
So a problem here is how the D2D Rx gets the timing info of the D2D TX for reception, while remaining in RRC_IDLE.